Future fire-driven landscape changes along a southwestern US elevation gradient

Over the 21st century, the combined effects of increased fire activity and
climate changes are expected to altered forest composition and structure
in many ecosystems by changing post-fire successional trajectories and
recovery. The southwestern US mountains encompass varied vegetation types
and species according to elevation which do not respond the same to
changing climate and fire regime. Moreover, fire exclusion applied during
the early 20th century has altered forest structure and fuel loads
compared to their natural states (i.e. without fire suppression).
Consequently, uncertainties persist about future vegetation shifts along
the elevation gradient. In this study, we simulated future vegetation
dynamics along an elevation gradient in the southwestern US comprising
pinyon-juniper woodlands, ponderosa pine forests and mixed conifer forests
for the period 2000-2099, to quantify the effects of future climate
conditions and projected wildfires on species productivity and
distribution. While we expected to find larger changes at low elevation
due to warmer and drier conditions, the largest changes occurred at high
elevation in mixed-conifer forests and were caused by wildfire. The
largest increase in high-severity and large fires were recorded in this
vegetation type, leading to high mortality of the dominant species, Picea
engelmannii and Abies lasiocarpa, which are not adapted to fire. The loss
of these two species reduced biomass productivity at high elevation. In
ponderosa pine forests and pinyon-juniper woodlands, fewer vegetation
changes occurred due to higher abundance of well-adapted species to fire
and the lower fuel loads mitigating projected fire activity, respectively.
Thus, future research should prioritize understanding of the processes
involved in future vegetation shifts in mixed-conifer forests in order to
mitigate the risk of loss of diversity specific to high-elevation forests
and the decrease in biomass productivity, and thus carbon storage
capacity, of these ecosystems due to wildfires.